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<h1 id="1-jsmn"><a href="#1-jsmn" class="headerlink" title="1. jsmn"></a>1. jsmn</h1><p>本期给大家带来的开源项目是 jsmn，<strong>一个资源占用极小的json解析器，号称世界上最快</strong>，作者zserge，目前收获 2.1K 个 star，遵循 MIT 开源许可协议。</p>
<p>jsmn主要有以下特性：</p>
<ul>
<li>没有任何库依赖关系；</li>
<li>语法与C89兼容，代码可移植性高；</li>
<li>没有任何动态内存分配</li>
<li>极小的代码占用</li>
<li>API只有两个，极其简洁</li>
</ul>
<blockquote>
<p>项目地址：<a href="https://github.com/zserge/jsmn" target="_blank" rel="noopener">https://github.com/zserge/jsmn</a></p>
</blockquote>
<h1 id="2-移植jsmn"><a href="#2-移植jsmn" class="headerlink" title="2. 移植jsmn"></a>2. 移植jsmn</h1><h2 id="2-1-移植思路"><a href="#2-1-移植思路" class="headerlink" title="2.1. 移植思路"></a>2.1. 移植思路</h2><p>开源项目在移植过程中主要参考项目的readme文档，一般只需两步：</p>
<ul>
<li>① 添加源码到裸机工程中；</li>
<li>② 实现需要的接口；</li>
</ul>
<h2 id="2-2-准备裸机工程"><a href="#2-2-准备裸机工程" class="headerlink" title="2.2. 准备裸机工程"></a>2.2. 准备裸机工程</h2><p>本文中我使用的是小熊派IoT开发套件，主控芯片为STM32L431RCT6：<br><img src="https://imgconvert.csdnimg.cn/aHR0cDovL21jdWxvdmVyNjY2LmNuL2Jsb2cvMjAxOTA5MTkvekh4NXhNWVVtU1cxLnBuZw?x-oss-process=image/format,png#pic_center" alt=""></p>
<p>移植之前需要准备一份裸机工程，我使用STM32CubeMX生成，需要初始化以下配置：</p>
<ul>
<li>配置一个串口用于发送数据；</li>
<li>printf重定向</li>
</ul>
<p>具体过程可以参考：</p>
<ul>
<li><a href="http://www.mculover666.cn/posts/1803605667/">STM32CubeMX_07 | 使用USART发送和接收数据（中断模式）</a></li>
<li><a href="http://www.mculover666.cn/posts/2251182441/">STM32CubeMX_09 | 重定向printf函数到串口输出的多种方法</a></li>
</ul>
<h1 id="2-3-添加jsmn到工程中"><a href="#2-3-添加jsmn到工程中" class="headerlink" title="2.3. 添加jsmn到工程中"></a>2.3. 添加jsmn到工程中</h1><p>① 复制jsmn源码到工程中：<br><img src="https://img-blog.csdnimg.cn/20200519112146427.png" alt=""></p>
<p>② 将 jsmn.h 文件添加到keil中（<strong>没有实质作用，方便编辑</strong>）：<br><img src="https://img-blog.csdnimg.cn/20200522141838534.png" alt=""><br>③ 添加jsmn头文件路径：<br><img src="https://img-blog.csdnimg.cn/20200522141951848.png" alt=""></p>
<h1 id="3-使用jsmn解析json数据"><a href="#3-使用jsmn解析json数据" class="headerlink" title="3. 使用jsmn解析json数据"></a>3. 使用jsmn解析json数据</h1><h2 id="3-1-准备工作"><a href="#3-1-准备工作" class="headerlink" title="3.1. 准备工作"></a>3.1. 准备工作</h2><p>① 包含jsmn头文件<br>使用时包含头文件，因为jsmn的函数定义也是在头文件中，所以第一次添加的时候，可以直接添加：</p>
<pre class=" language-c"><code class="language-c"><span class="token comment" spellcheck="true">/* USER CODE BEGIN Includes */</span>
<span class="token macro property">#<span class="token directive keyword">include</span> <span class="token string">"jsmn.h"</span></span>
<span class="token macro property">#<span class="token directive keyword">include</span> <span class="token string">&lt;stdio.h></span>    </span><span class="token comment" spellcheck="true">//用于printf打印</span>
<span class="token macro property">#<span class="token directive keyword">include</span> <span class="token string">&lt;string.h></span> </span><span class="token comment" spellcheck="true">//用于字符串处理</span>

<span class="token comment" spellcheck="true">/* USER CODE END Includes */</span></code></pre>
<p>已经使用过之后，在别的文件中继续使用时，需要这样添加，且<strong>顺序不可互换</strong>：</p>
<pre class=" language-c"><code class="language-c"><span class="token comment" spellcheck="true">/* USER CODE BEGIN 0 */</span>
<span class="token macro property">#<span class="token directive keyword">define</span> JSMN_HEADER</span>
<span class="token macro property">#<span class="token directive keyword">include</span> <span class="token string">"jsmn.h"</span>    </span>

<span class="token comment" spellcheck="true">/* USER CODE END 0 */</span></code></pre>
<p>否则会造成函数重定义：<br><img src="https://img-blog.csdnimg.cn/2020052309101761.png" alt=""></p>
<p>② 设置一段原始json数据<br>在main.c中设置原始的json数据，用于后续解析：</p>
<pre class=" language-c"><code class="language-c"><span class="token comment" spellcheck="true">/* USER CODE BEGIN PV */</span>
<span class="token keyword">static</span> <span class="token keyword">const</span> <span class="token keyword">char</span> <span class="token operator">*</span>JSON_STRING <span class="token operator">=</span>
    <span class="token string">"{\"user\": \"johndoe\", \"admin\": false, \"uid\": 1000,\n"</span>
    <span class="token string">"\"groups\": [\"users\", \"wheel\", \"audio\", \"video\"]}"</span><span class="token punctuation">;</span>
<span class="token comment" spellcheck="true">/* USER CODE END PV */</span></code></pre>
<p>③ 开辟一块存放token的数组（token池）<br>jsmn中，每个数据段解析出来之后是一个token，关于token的详细解释，请参考下文第4.1小节。</p>
<pre class=" language-c"><code class="language-c"><span class="token comment" spellcheck="true">/* USER CODE BEGIN PV */</span>

jsmntok_t t<span class="token punctuation">[</span><span class="token number">128</span><span class="token punctuation">]</span><span class="token punctuation">;</span>

<span class="token comment" spellcheck="true">/* USER CODE END PV */</span></code></pre>
<p>④ 编写在原始JSON数据中的字符串比较函数：</p>
<pre class=" language-c"><code class="language-c"><span class="token keyword">static</span> <span class="token keyword">int</span> <span class="token function">jsoneq</span><span class="token punctuation">(</span><span class="token keyword">const</span> <span class="token keyword">char</span> <span class="token operator">*</span>json<span class="token punctuation">,</span> jsmntok_t <span class="token operator">*</span>tok<span class="token punctuation">,</span> <span class="token keyword">const</span> <span class="token keyword">char</span> <span class="token operator">*</span>s<span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">if</span> <span class="token punctuation">(</span>tok<span class="token operator">-></span>type <span class="token operator">==</span> JSMN_STRING <span class="token operator">&amp;&amp;</span> <span class="token punctuation">(</span><span class="token keyword">int</span><span class="token punctuation">)</span><span class="token function">strlen</span><span class="token punctuation">(</span>s<span class="token punctuation">)</span> <span class="token operator">==</span> tok<span class="token operator">-></span>end <span class="token operator">-</span> tok<span class="token operator">-></span>start <span class="token operator">&amp;&amp;</span>
      <span class="token function">strncmp</span><span class="token punctuation">(</span>json <span class="token operator">+</span> tok<span class="token operator">-></span>start<span class="token punctuation">,</span> s<span class="token punctuation">,</span> tok<span class="token operator">-></span>end <span class="token operator">-</span> tok<span class="token operator">-></span>start<span class="token punctuation">)</span> <span class="token operator">==</span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> <span class="token number">0</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
  <span class="token keyword">return</span> <span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span></code></pre>
<h2 id="3-2-创建并初始化解析器"><a href="#3-2-创建并初始化解析器" class="headerlink" title="3.2. 创建并初始化解析器"></a>3.2. 创建并初始化解析器</h2><p>在main函数的开始创建解析器：</p>
<pre class=" language-c"><code class="language-c"><span class="token comment" spellcheck="true">/* USER CODE BEGIN 1 */</span>
    <span class="token keyword">int</span> r<span class="token punctuation">;</span>
    <span class="token keyword">int</span> i<span class="token punctuation">;</span>

    jsmn_parser p<span class="token punctuation">;</span><span class="token comment" spellcheck="true">//jsmn解析器</span>

<span class="token comment" spellcheck="true">/* USER CODE END 1 */</span></code></pre>
<p>在随后外设初始化完成之后的代码中初始化解析器：</p>
<pre class=" language-c"><code class="language-c"><span class="token comment" spellcheck="true">/* USER CODE BEGIN 2 */</span>

    <span class="token function">jsmn_init</span><span class="token punctuation">(</span><span class="token operator">&amp;</span>p<span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token comment" spellcheck="true">/* USER CODE END 2 */</span></code></pre>
<h2 id="3-3-解析数据，获取token"><a href="#3-3-解析数据，获取token" class="headerlink" title="3.3. 解析数据，获取token"></a>3.3. 解析数据，获取token</h2><pre class=" language-c"><code class="language-c">r <span class="token operator">=</span> <span class="token function">jsmn_parse</span><span class="token punctuation">(</span><span class="token operator">&amp;</span>p<span class="token punctuation">,</span> JSON_STRING<span class="token punctuation">,</span> <span class="token function">strlen</span><span class="token punctuation">(</span>JSON_STRING<span class="token punctuation">)</span><span class="token punctuation">,</span> t<span class="token punctuation">,</span><span class="token keyword">sizeof</span><span class="token punctuation">(</span>t<span class="token punctuation">)</span> <span class="token operator">/</span> <span class="token keyword">sizeof</span><span class="token punctuation">(</span>t<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

  <span class="token keyword">if</span> <span class="token punctuation">(</span>r <span class="token operator">&lt;</span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token function">printf</span><span class="token punctuation">(</span><span class="token string">"Failed to parse JSON: %d\n"</span><span class="token punctuation">,</span> r<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">return</span> <span class="token number">1</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>

  <span class="token comment" spellcheck="true">/* Assume the top-level element is an object */</span>
  <span class="token keyword">if</span> <span class="token punctuation">(</span>r <span class="token operator">&lt;</span> <span class="token number">1</span> <span class="token operator">||</span> t<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span><span class="token punctuation">.</span>type <span class="token operator">!=</span> JSMN_OBJECT<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token function">printf</span><span class="token punctuation">(</span><span class="token string">"Object expected\n"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">return</span> <span class="token number">1</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span></code></pre>
<h2 id="3-4-逐个解析token"><a href="#3-4-逐个解析token" class="headerlink" title="3.4. 逐个解析token"></a>3.4. 逐个解析token</h2><pre class=" language-c"><code class="language-c"><span class="token comment" spellcheck="true">/* Loop over all keys of the root object */</span>
 <span class="token keyword">for</span> <span class="token punctuation">(</span>i <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span> i <span class="token operator">&lt;</span> r<span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> 
 <span class="token punctuation">{</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token function">jsoneq</span><span class="token punctuation">(</span>JSON_STRING<span class="token punctuation">,</span> <span class="token operator">&amp;</span>t<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token punctuation">,</span> <span class="token string">"user"</span><span class="token punctuation">)</span> <span class="token operator">==</span> <span class="token number">0</span><span class="token punctuation">)</span>
    <span class="token punctuation">{</span>
          <span class="token comment" spellcheck="true">/* We may use strndup() to fetch string value */</span>
          <span class="token function">printf</span><span class="token punctuation">(</span><span class="token string">"- user: %.*s\n"</span><span class="token punctuation">,</span> t<span class="token punctuation">[</span>i <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">]</span><span class="token punctuation">.</span>end <span class="token operator">-</span> t<span class="token punctuation">[</span>i <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">]</span><span class="token punctuation">.</span>start<span class="token punctuation">,</span>
             JSON_STRING <span class="token operator">+</span> t<span class="token punctuation">[</span>i <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">]</span><span class="token punctuation">.</span>start<span class="token punctuation">)</span><span class="token punctuation">;</span>
          i<span class="token operator">++</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
    <span class="token keyword">else</span> <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token function">jsoneq</span><span class="token punctuation">(</span>JSON_STRING<span class="token punctuation">,</span> <span class="token operator">&amp;</span>t<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token punctuation">,</span> <span class="token string">"admin"</span><span class="token punctuation">)</span> <span class="token operator">==</span> <span class="token number">0</span><span class="token punctuation">)</span> 
    <span class="token punctuation">{</span>
          <span class="token comment" spellcheck="true">/* We may additionally check if the value is either "true" or "false" */</span>
          <span class="token function">printf</span><span class="token punctuation">(</span><span class="token string">"- Admin: %.*s\n"</span><span class="token punctuation">,</span> t<span class="token punctuation">[</span>i <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">]</span><span class="token punctuation">.</span>end <span class="token operator">-</span> t<span class="token punctuation">[</span>i <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">]</span><span class="token punctuation">.</span>start<span class="token punctuation">,</span>
             JSON_STRING <span class="token operator">+</span> t<span class="token punctuation">[</span>i <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">]</span><span class="token punctuation">.</span>start<span class="token punctuation">)</span><span class="token punctuation">;</span>
          i<span class="token operator">++</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
    <span class="token keyword">else</span> <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token function">jsoneq</span><span class="token punctuation">(</span>JSON_STRING<span class="token punctuation">,</span> <span class="token operator">&amp;</span>t<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token punctuation">,</span> <span class="token string">"uid"</span><span class="token punctuation">)</span> <span class="token operator">==</span> <span class="token number">0</span><span class="token punctuation">)</span> 
    <span class="token punctuation">{</span>
          <span class="token comment" spellcheck="true">/* We may want to do strtol() here to get numeric value */</span>
          <span class="token function">printf</span><span class="token punctuation">(</span><span class="token string">"- UID: %.*s\n"</span><span class="token punctuation">,</span> t<span class="token punctuation">[</span>i <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">]</span><span class="token punctuation">.</span>end <span class="token operator">-</span> t<span class="token punctuation">[</span>i <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">]</span><span class="token punctuation">.</span>start<span class="token punctuation">,</span>
             JSON_STRING <span class="token operator">+</span> t<span class="token punctuation">[</span>i <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">]</span><span class="token punctuation">.</span>start<span class="token punctuation">)</span><span class="token punctuation">;</span>
          i<span class="token operator">++</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
    <span class="token keyword">else</span> <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token function">jsoneq</span><span class="token punctuation">(</span>JSON_STRING<span class="token punctuation">,</span> <span class="token operator">&amp;</span>t<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token punctuation">,</span> <span class="token string">"groups"</span><span class="token punctuation">)</span> <span class="token operator">==</span> <span class="token number">0</span><span class="token punctuation">)</span> 
    <span class="token punctuation">{</span>
          <span class="token keyword">int</span> j<span class="token punctuation">;</span>
          <span class="token function">printf</span><span class="token punctuation">(</span><span class="token string">"- Groups:\n"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
          <span class="token keyword">if</span> <span class="token punctuation">(</span>t<span class="token punctuation">[</span>i <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">]</span><span class="token punctuation">.</span>type <span class="token operator">!=</span> JSMN_ARRAY<span class="token punctuation">)</span> 
          <span class="token punctuation">{</span>
            <span class="token keyword">continue</span><span class="token punctuation">;</span> <span class="token comment" spellcheck="true">/* We expect groups to be an array of strings */</span>
          <span class="token punctuation">}</span>
          <span class="token keyword">for</span> <span class="token punctuation">(</span>j <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> j <span class="token operator">&lt;</span> t<span class="token punctuation">[</span>i <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">]</span><span class="token punctuation">.</span>size<span class="token punctuation">;</span> j<span class="token operator">++</span><span class="token punctuation">)</span> 
          <span class="token punctuation">{</span>
            jsmntok_t <span class="token operator">*</span>g <span class="token operator">=</span> <span class="token operator">&amp;</span>t<span class="token punctuation">[</span>i <span class="token operator">+</span> j <span class="token operator">+</span> <span class="token number">2</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
            <span class="token function">printf</span><span class="token punctuation">(</span><span class="token string">"  * %.*s\n"</span><span class="token punctuation">,</span> g<span class="token operator">-></span>end <span class="token operator">-</span> g<span class="token operator">-></span>start<span class="token punctuation">,</span> JSON_STRING <span class="token operator">+</span> g<span class="token operator">-></span>start<span class="token punctuation">)</span><span class="token punctuation">;</span>
          <span class="token punctuation">}</span>
          i <span class="token operator">+</span><span class="token operator">=</span> t<span class="token punctuation">[</span>i <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">]</span><span class="token punctuation">.</span>size <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
    <span class="token keyword">else</span>
    <span class="token punctuation">{</span>
          <span class="token function">printf</span><span class="token punctuation">(</span><span class="token string">"Unexpected key: %.*s\n"</span><span class="token punctuation">,</span> t<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token punctuation">.</span>end <span class="token operator">-</span> t<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token punctuation">.</span>start<span class="token punctuation">,</span>
             JSON_STRING <span class="token operator">+</span> t<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token punctuation">.</span>start<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
  <span class="token punctuation">}</span></code></pre>
<h2 id="3-5-解析结果"><a href="#3-5-解析结果" class="headerlink" title="3.5. 解析结果"></a>3.5. 解析结果</h2><p>编译、下载到开发板，使用串口助手进行测试：<br><img src="https://img-blog.csdnimg.cn/20200522155008406.png" alt=""></p>
<h2 id="3-6-内存对比"><a href="#3-6-内存对比" class="headerlink" title="3.6. 内存对比"></a>3.6. 内存对比</h2><p><img src="https://img-blog.csdnimg.cn/20200522154854214.png" alt=""></p>
<h1 id="4-jsmn设计思想解读"><a href="#4-jsmn设计思想解读" class="headerlink" title="4. jsmn设计思想解读"></a>4. jsmn设计思想解读</h1><h2 id="4-1-jsmn对json数据项的抽象"><a href="#4-1-jsmn对json数据项的抽象" class="headerlink" title="4.1. jsmn对json数据项的抽象"></a>4.1. jsmn对json数据项的抽象</h2><p>jsmn对json数据中的每一个数据段都会抽象为一个结构体，称之为token，此结构体非常简洁：</p>
<pre class=" language-c"><code class="language-c"><span class="token comment" spellcheck="true">/**
 * JSON token description.
 * type        type (object, array, string etc.)
 * start    start position in JSON data string
 * end        end position in JSON data string
 */</span>
<span class="token keyword">typedef</span> <span class="token keyword">struct</span> jsmntok <span class="token punctuation">{</span>
  jsmntype_t type<span class="token punctuation">;</span>
  <span class="token keyword">int</span> start<span class="token punctuation">;</span>
  <span class="token keyword">int</span> end<span class="token punctuation">;</span>
  <span class="token keyword">int</span> size<span class="token punctuation">;</span>
<span class="token macro property">#<span class="token directive keyword">ifdef</span> JSMN_PARENT_LINKS</span>
  <span class="token keyword">int</span> parent<span class="token punctuation">;</span>
<span class="token macro property">#<span class="token directive keyword">endif</span></span>
<span class="token punctuation">}</span> jsmntok_t<span class="token punctuation">;</span></code></pre>
<p>在本实验中未开启<code>JSMN_PARENT_LINKS</code>，所以此结构体占用<strong>16Byte大小</strong>。</p>
<p>从结构体中的数据成员可以看出，<strong>jsmn并不保存任何具体的数据内容</strong>，仅仅记录：</p>
<ul>
<li>数据项的类型</li>
<li>数据项数据段在原始json数据中的起始位置</li>
<li>数据项数据段在原始json数据中的结束位置</li>
</ul>
<p>其中，数据项的类型支持4种：</p>
<pre class=" language-c"><code class="language-c"><span class="token comment" spellcheck="true">/**
 * JSON type identifier. Basic types are:
 *     o Object
 *     o Array
 *     o String
 *     o Other primitive: number, boolean (true/false) or null
 */</span>
<span class="token keyword">typedef</span> <span class="token keyword">enum</span> <span class="token punctuation">{</span>
  JSMN_UNDEFINED <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">,</span>
  JSMN_OBJECT <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">,</span>
  JSMN_ARRAY <span class="token operator">=</span> <span class="token number">2</span><span class="token punctuation">,</span>
  JSMN_STRING <span class="token operator">=</span> <span class="token number">3</span><span class="token punctuation">,</span>
  JSMN_PRIMITIVE <span class="token operator">=</span> <span class="token number">4</span>
<span class="token punctuation">}</span> jsmntype_t<span class="token punctuation">;</span></code></pre>
<h2 id="4-2-jsmn如何解析出每个token"><a href="#4-2-jsmn如何解析出每个token" class="headerlink" title="4.2. jsmn如何解析出每个token"></a>4.2. jsmn如何解析出每个token</h2><p>上述说到jsmn将每一个json数据段都抽象为一个token，那么jsmn是如何对整段json数据进行解析，得到每一个数据项的token呢？</p>
<p>jsmn解析器也是非常简洁的一个结构体：</p>
<pre class=" language-c"><code class="language-c"><span class="token comment" spellcheck="true">/**
 * JSON parser. Contains an array of token blocks available. Also stores
 * the string being parsed now and current position in that string.
 */</span>
<span class="token keyword">typedef</span> <span class="token keyword">struct</span> jsmn_parser <span class="token punctuation">{</span>
  <span class="token keyword">unsigned</span> <span class="token keyword">int</span> pos<span class="token punctuation">;</span>     <span class="token comment" spellcheck="true">/* offset in the JSON string */</span>
  <span class="token keyword">unsigned</span> <span class="token keyword">int</span> toknext<span class="token punctuation">;</span> <span class="token comment" spellcheck="true">/* next token to allocate */</span>
  <span class="token keyword">int</span> toksuper<span class="token punctuation">;</span>         <span class="token comment" spellcheck="true">/* superior token node, e.g. parent object or array */</span>
<span class="token punctuation">}</span> jsmn_parser<span class="token punctuation">;</span></code></pre>
<p>jsmn解析就是<strong>将json数据逐个字符进行解析</strong>，用pos数据成员来记录解析器当前的位置，当寻找到特殊字符时，就去之前我们定义的token数组（t）中申请一个空的token成员，将该token在数组中的位置记录在数据成员toknext中。</p>
<p>源码在下面的函数中，代码过多，暂且先不放：</p>
<pre class=" language-c"><code class="language-c">JSMN_API <span class="token keyword">int</span> <span class="token function">jsmn_parse</span><span class="token punctuation">(</span>jsmn_parser <span class="token operator">*</span>parser<span class="token punctuation">,</span> <span class="token keyword">const</span> <span class="token keyword">char</span> <span class="token operator">*</span>js<span class="token punctuation">,</span> <span class="token keyword">const</span> size_t len<span class="token punctuation">,</span>
                        jsmntok_t <span class="token operator">*</span>tokens<span class="token punctuation">,</span> <span class="token keyword">const</span> <span class="token keyword">unsigned</span> <span class="token keyword">int</span> num_tokens<span class="token punctuation">)</span><span class="token punctuation">;</span></code></pre>
<p>下面用一个实例来看看token是怎么分配的。</p>
<p>缩短json原始数据：</p>
<pre class=" language-c"><code class="language-c"><span class="token keyword">static</span> <span class="token keyword">const</span> <span class="token keyword">char</span> <span class="token operator">*</span>JSON_STRING <span class="token operator">=</span>
    <span class="token string">"{\"name\":\"mculover666\",\"admin\":false,\"uid\":1000}"</span><span class="token punctuation">;</span></code></pre>
<p>在解析之后将每个token打印出来：</p>
<pre class=" language-c"><code class="language-c"><span class="token function">printf</span><span class="token punctuation">(</span><span class="token string">"[type][start][end][size]\n"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">for</span><span class="token punctuation">(</span>i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>i <span class="token operator">&lt;</span> r<span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span>
<span class="token punctuation">{</span>
    <span class="token function">printf</span><span class="token punctuation">(</span><span class="token string">"[%4d][%5d][%3d][%4d]\n"</span><span class="token punctuation">,</span> t<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token punctuation">.</span>type<span class="token punctuation">,</span> t<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token punctuation">.</span>start<span class="token punctuation">,</span> t<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token punctuation">.</span>end<span class="token punctuation">,</span> t<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token punctuation">.</span>size<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span></code></pre>
<p>结果如下：<br><img src="https://img-blog.csdnimg.cn/20200522154032155.png" alt=""><br>这段json数据解析出的token有7个：</p>
<p>① Object类型的token：<code>{\&quot;name\&quot;:\&quot;mculover666\&quot;,\&quot;admin\&quot;:false,\&quot;uid\&quot;:1000}</code><br>② String类型的token：<code>&quot;name&quot;</code>、<code>&quot;mculover666&quot;</code>、<code>&quot;admin&quot;</code>、<code>&quot;uid&quot;</code><br>③ Primitive类型的token：数字<code>1000</code>，布尔值<code>false</code></p>
<p><img src="https://img-blog.csdnimg.cn/20200522153959263.png" alt=""></p>
<h2 id="4-3-用户如何从token中提取值"><a href="#4-3-用户如何从token中提取值" class="headerlink" title="4.3.  用户如何从token中提取值"></a>4.3.  用户如何从token中提取值</h2><p>在解析完毕获得这些token之后，需要根据token数量来判断是否解析成功：</p>
<p>① 返回的token数量&lt;0：证明解析失败，返回值代表了错误类型：</p>
<pre class=" language-c"><code class="language-c"><span class="token keyword">enum</span> jsmnerr <span class="token punctuation">{</span>
  <span class="token comment" spellcheck="true">/* Not enough tokens were provided */</span>
  JSMN_ERROR_NOMEM <span class="token operator">=</span> <span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">,</span>
  <span class="token comment" spellcheck="true">/* Invalid character inside JSON string */</span>
  JSMN_ERROR_INVAL <span class="token operator">=</span> <span class="token operator">-</span><span class="token number">2</span><span class="token punctuation">,</span>
  <span class="token comment" spellcheck="true">/* The string is not a full JSON packet, more bytes expected */</span>
  JSMN_ERROR_PART <span class="token operator">=</span> <span class="token operator">-</span><span class="token number">3</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span></code></pre>
<p>② 判断第0个token是否是JSMN_OBJECT类型，如果不是，则证明解析错误。</p>
<p>③ 如果token数量大于1，则从第1个token开始判断<strong>字符串是否与给定的键值对的名称相等</strong>，若相等，则提取下一个token的内容作为该键值对的值。</p>
<h1 id="5-项目工程源码获取和问题交流"><a href="#5-项目工程源码获取和问题交流" class="headerlink" title="5. 项目工程源码获取和问题交流"></a>5. 项目工程源码获取和问题交流</h1><p>目前我将jsmn源码、我移植到小熊派STM32L431RCT6开发板的工程源码上传到了QQ群里（包含好几份HAL库，QQ相对速度快点），<strong>可以在QQ群里下载，有问题也可以在群里交流，当然也欢迎大家分享出来自己移植的工程到QQ群里</strong>：<br><img src="https://img-blog.csdnimg.cn/20200516103153959.png" alt=""></p>
<p>放上QQ群二维码：<br><img src="https://img-blog.csdnimg.cn/20200321144543508.png" alt=""></p>
<p><font color="red"><strong>接收更多精彩文章及资源推送，欢迎订阅我的微信公众号：『mculover666』。</strong></font><br><img src="https://img-blog.csdnimg.cn/20200202092055136.png" alt=""></p>

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